This invention concerns inorganic fluorine chemistry and more particularly concerns a process by means of which sulfur tetrafluoride, sulfur chloride pentafluoride and sulfur hexafluoride may be prepared with the use of little or no elemental fluorine as a reactant.
Heretofore, sulfur tetrafluoride (SF.sub.4), sulfur chloride pentafluoride (SF.sub.5 Cl) and sulfur hexafluoride (SF.sub.6) have been prepared by means of processes which utilize direct fluorination of sulfur.
Sulfur hexafluoride is a particularly desirable compound since it is almost totally inert, being unaffected by aqueous or fused alkali, ammonia or oxygen and even alkali metals react appreciably only at elevated temperatures. In addition, sulfur hexafluoride has a high dielectric strength which has led to the use of sulfur hexafluoride as an insulating atmosphere for high voltage electrical equipment.
Sulfur tetrafluoride has particular utility as an intermediate to the preparation of sulfur hexafluoride and as an intermediate in the preparation of many other fluorine containing compounds. For example, sulfur tetrafluoride reacts with compounds containing carboxylic and sulfonic acid groups to form acid fluorides; reacts with organic compounds containing a carbon containing group such as aldehydes and ketones to replace the oxygen atom by two fluorine atoms, even in the presence of unsaturated carbon atoms; reacts with most inorganic oxides and sulfides to give corresponding fluorides or oxyfluorides; reacts with transition metal oxides or sulfides to obtain corresponding transition metal fluorides and reacts with organic chlorides in the presence of boron trichloride to yield corresponding fluorides.
Sulfur chloride pentafluoride reacts with itself at about 400.degree. C to yield sulfur hexafluoride, sulfur tetrafluoride and chlorine. As previously discussed, the resulting sulfur hexafluoride has substantial utility due to its inert nature and high dielectric strength. In addition, sulfur chloride pentafluoride is a powerful oxidizing agent and oxidizes alcohols and aldehydes to carboxylic acids and aromatic amines to azo compounds. Sulfur chloride pentafluoride can add across the double or triple bonds to olefins and acetylenes to obtain carbon compounds to which chlorine and SF.sub.5 has been incorporated through addition.
While the foregoing compounds, i.e., sulfur hexafluoride, sulfur tetrafluoride and sulfur chloride pentafluoride have a large amount of utility, commercially, they have been prepared only by means of processes which have serious disadvantages. Such fluoride containing compounds were prepared in the prior art through the use of elemental fluorine, expensive reaction solvents, expensive alkali metal fluorides, or through electrochemical fluorination. A process for preparing a fluorine containing compound which requires the use of elemental fluorine is desirably avoided since the preparation of elemental fluorine requires fluorine generators which are operated only with difficulty. Such fluorine generators due to the extremely high chemical reactivity of fluorine, must be manufactured of costly materials of construction in an attempt to extend the life of the generators. Even when the best available materials of construction are employed in the fluorine generators, the fluorine attacks the materials of construction, thus requiring that parts of the generators be frequently replaced. In addition, elemental fluorine must be carefully handled since inadvertent leaks or mixing fluorine with other compounds or compositions can result in serious explosions. Since elemental fluorine is to be avoided in the preparation of sulfur hexafluoride, sulfur tetrafluoride and sulfur chloride pentafluoride, attempts have been made in the prior art to prepare sulfur fluorides, particularly sulfur tetrafluoride, without using elemental fluorine. Such proposed processes for preparing sulfur fluorides have not been entirely satisfactory since the processes required expensive reaction solvents, utilized expensive alkali metal fluorides which could be economically regenerated or recovered, or utilized electro-chemical fluorination which requires costly and sophisticated processing equipment.
Additionally, prior art processes for preparing the previously mentioned sulfur fluorides did not permit recycling and regeneration of the reactants which were more difficult to prepare and more costly to use.
A prior art process for producing thionly fluoride, SOF.sub.2, from sulfur chlorides and NOF or NOF.nHF is disclosed in U.S. Pat. No. 3,074,781. The failure of the disclosed process to have produced SF.sub.4 is believed to be due to the lack of anhydrous conditions such that any SF.sub.4 that might have been formed during the experiments reported therein would have been hydrolyzed to SOF.sub.2 by water remaining in the system or, more likely, in the distillation and sampling steps that followed.